Could a cooler sun, which some solar astronomers now predict, save us from global warming?
The short answer is "no," scientists say.
"That would be convenient and would make a lot of people happy," said Greg Kopp, a physicist with the Laboratory for Atmospheric and Space Physics at the University of Colorado.
Kopp notes that the sun has been in a quiet phase during the last decade's rise in global temperatures.
"Human-caused climate effects are far outweighing the solar effects currently," said Kopp, who is an instrument scientist for a NASA satellite that measures the sun's heat output, or total solar irradiance.
Satellites have measured the sun's radiation at the point where it reaches Earth's atmosphere for 32 years. The average difference between energy at the peak and minimum of solar cycles is less than 0.1 percent.
That flux is easily absorbed and balanced by the Earth's oceans, Kopp said, and has a minimal effect on the Earth's temperature.
That averaging would not occur if the sun stayed cool for a prolonged period, and some solar astronomers see that approaching.
NASA predicts that the current solar cycle will have the lowest maximum phase in 200 years, and astronomers with the National Solar Observatory see signs that the following cycle will be just as weak.
Some astronomers predict a phase similar to the Maunder Minimum, when an absence of sunspots in significant numbers was recorded for 70 years.
That period, between 1645 A.D. and 1715 A.D., coincided with colder temperatures in Europe during a longer period that is sometimes called the "Little Ice Age."
Scientists have known for centuries that sunspots appear in greater numbers in a regular cycle that averages about 11 years.
The sunspots correlate with increased eruptions from the sun's surface and energy escaping it.
Solar Cycle 23 took about 13 years to complete and was marked by long periods of no sunspots. Cycle 24, which began in January 2008, is predicted to be even weaker.
Frank Hill, associate director for synoptic systems at the National Solar Observatory in Tucson, said the current solar cycle and the following one will be weak. A continuation "is not out of the question."
Hill's Global Oscillation Network Group (GONG) - six instruments spaced across the globe - uses sound waves to take sonogramlike pictures of the sun.
GONG has uncovered "jet streams" deep within the sun that migrate from near the polls to the equator. Sunspots appear when those streams reach a latitude of 22 degrees.
The streams and the sunspots were slow to arrive for Cycle 24. Observations now indicate Cycle 25 will be similar, Hill said.
At the National Solar Observatory, Matthew Penn and William Livingston use a different measurement but reach a similar conclusion.
For 17 years, Livingston has used the McMath-Pierce solar telescope on Kitt Peak to observe sunspots' weakening magnetic field. If the trend continued, they would disappear by the end of this decade.
Penn and Livingston's first paper on the subject created controversy in 2006, when it proposed a possible recurrence of the Maunder Minimum and was used by those opposing greenhouse-gas caps to argue that human-caused changes in climate are insignificant.
Hill doesn't think the two subjects should be linked. "My feeling is that the solar activity cycle has nothing to do with global warming."
Mark Giampapa, the solar observatory's deputy director, said his studies of sunlike stars convinced him that prolonged periods of inactivity may be a normal occurrence for our sun. "We've had an unusually active sun for much of the 20th century," he said.
That doesn't mean we are headed for an ice age or that we needn't worry about global warming, said climate scientist Julia Cole.
"The sun's influence on climate is a really important research topic," said Cole, a professor of geosciences and atmospheric sciences at the University of Arizona. But while much is unknown, the effect of solar flux is small in comparison to greenhouse gases.
The sun may have had a role in the cold weather recorded during the Maunder Minimum, she said, but the cooling was geographically specific, "primarily defined by records from Europe - it wasn't a global chill," she said.
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